The invention relates to a method and apparatus for the continuous pressure comminution of brittle grinding stock such as cement clinker, ore, coal or the like.
More particularly, the method relates to stock preparation using a high-pressure roller press in which in the narrowest nip the grinding stock is subjected to a product bed comminution. The comminuted, pressed stock is de-agglomerated, and the adequately fine finished product is sorted from the de-agglomerated stock in a sifter. The coarse fraction (grit) is recirculated for redelivery to the roller press.
Considerable efforts have been made recently to increase the low energy exploitation in comminution and grinding systems, particularly those comprising tube mills or ball mills. A method for the comminution and grinding of brittle grinding stock has been disclosed in European Patent No. 0 084 383, for example in FIG. 2, wherein grinding stock, for example non-pre-comminuted cement clinker chunks, is pressed under high pressure in a first step in the nip of a high-pressure roller press. This leads to particle self-destruction, partly due to the creation of incipient cracks in the inside of the particles and being visibly expressed in the formation of agglomerates (what are referred to as scabs). These are then de-agglomerated or destroyed in the second step in a tube mill or ball mill which have a comparatively low energy outlay and can be ground to finished product fineness. This is because the grindability of the agglomerates produced by the pressing is considerably improved in comparison to unpressed material. In the region of the narrowest nip of the high-pressure roller press, the individual particles of the grinding stock are mutually crushed in a product bed, i.e., in a bulk material compressed between two surfaces. This is what is referred to as product bed comminution.
Product bed comminution is different and distinctive from mere roller pressing. In product bed comminution, the material to be crushed is supplied to the roller gap in a large quantity such that the product to be comminuted is drawn in between rollers and presses the rollers apart so that the particles of feed mutually crush one another in the roller gap to produce an agglomerated product bed. The gap width of the rollers resiliently pressed against each other greater than the particle size of the feed is maintained. Various arrangements have been attempted utilizing this unique process and operation and one form of utilization is referred to and described in U.S. Pat. No. 4,703,897, Beisner et al, issued Nov. 3, 1987.
U.S. Pat. No. 4,726,531, issued Feb. 23, 1988, Strasser, discloses a method and mechanism employing product bed comminution. As discussed therein, the charging stock to be comminuted is supplied to the nip of a press in a large quantity via a vertical stack arranged above the nip and the particles of charging stock mutually crush one another in the nip. The particles have internal incipient cracks. When the cement clinker is ground, it emerges from the nip comminuted and partially agglomerated to form scabs. The scabs are of such a nature that they can be crumbled by hand and thereafter broken up and deagglomerated, and a proportion of particles will have already been reduced to the desired fineness such as on the order of about 30% having a fineness of less than 90 .mu.m.
Due to the high pressing power of the roller press upon destruction of the structure of the particles, for example, pressed granules of cement clinker, the agglomerates (scabs) that are formed constitute a certain proportion of particles that are already reduced to the desired cement fineness. This proportion that need not be further comminuted can burden the tube mill in the finished grinding of the agglomerates discharged from the roller press.
The energy exploitation or efficiency of tube mills or ball mills continues to be low and a roller press exhibits a lower specific energy consumption (kWh/t) in employment of product bed comminution in comparison to the ball mill. A method for comminution and grinding of brittle grinding stock such as the grinding of cement clinker to form cement is known (FIG. 4 of European Patent No. 0 084 383) wherein a tube mill or ball mill has been completely eliminated. The product pressed to form scabs in the nip of the roller press is thereby de-agglomerated and the finished cement is sorted out from the de-agglomerated product in a sifter, whereas the coarse fraction (grits) is recirculated for delivery to the roller press. When the adequately fine proportion of finished product contained in the pressed scabs of the roller press amounts to only 10% through 20%, then the grinding process must be operated with high circulation loads having a factor of up to more than 9. This means that, considered statistically, every particle of the product to be comminuted or, respectively, to be ground not only has to be scabbed about ten times but also has to be de-agglomerated, sifted and conveyed back to the roller press just as many times until it has arisen as finished product. The energy balance of the roller press itself that is initially very favorable is therefore diminished again because of frequent traversal of the product stream through the three sub-units of de-agglomerater, sifter and grits conveying means. This situation could be superficially improved in that a sub-stream of the scabs obtained from the roller press is directly returned into the delivery shaft of this roller press. However, heterogeneous pressure distributions--what are referred to as pressure islands--then arise in the nip of the roller press due to the different densities of fresh charging stock (for example 1600 kg/m.sup.3) and scabs (for example, 2400 kg/m.sup.3), so that the scabs experience the highest--potentially excessively high--pressure in the nip due to their pre-pressure, whereas the fresh charging stock receives too little pressure due to its greater porosity and is therefore inadequately pressed. Even given this interconnection of the roller press, the scabs that proceed into the sifter can thereby still contain a relatively great quantity of coarse product, as a result whereof the product circulation load must continue to be high. The surfaces of the roller jackets, moreover, can be over-stressed due to the formation of pressure islands in the nip.
An object of the invention is to develop and improve a comminution method employing a high-pressure roller press but without employing a following ball mill such that it is possible to grind cement clinker into finished cement with a high degree of comminution in an economical fashion.
It is a further object of the invention to provide an improved apparatus and method for comminuting brittle stock such as cement clinker utilizing the principles of product bed comminution and which have a reduced energy consumption decreasing the cost of production and reducing the cost of machinery installation.
It is also an object of the invention to provide an improved comminution installation utilizing the principles of the method.